Linkage in the X-chromosom Lab.

Linkage in the X-chromosom The females exhibit recombinations for the genes located in the sexchromosomes in the same manner as they do for the genes located in the autosomes. In male Drosophila no crossing over at all occurs. Interference andCoincidence: Is the quantitative estimate of the correspondents of double recombinants obtained to those expected

Linkage in the X-chromosom Coefficient of coincidence (CoC) % of double recombinant observed\% of double recombinants expected It has been found that in interference usually increases as the distance between loci becomes smaller, until a point is reached when no double crosses are found, CoC = 0. When the observed number of double recombinants is equal to the expected number, interference disappears and CoC is 1

Procedur 1. Set a cross with gene loci residing on one chromosome (e.g.: yellow, vermilion, forked). 2. Evaluate the F2-generation once available by recording the various combinations of the y, v, f and wild type genes and record the following: • wildtype (y+ ,v+ , f+ ) • the mutants (y, v, f), the recombiants of cross-overs (6 more classes) . 3. Determine the percentage of occurrence of each of the eight classes. 4. Calculate the percentage of crossing over or recombination between each pair of genes.

5. Draw a linkage map carrying the respective genes (mutants) based on the percentage of recombinants, with the distances between them in scale with the crossing-over values between them. 6. Compare the frequency of double crossover with that excepted. (CoC). 7. Record and explain the results. Results and Evaluation: see back of the tri-hybrid cross (yellow-vermilion-forked)data shee Procedur